[Bug]: "Make Default" button in Settings corrupts mimeapps.list on Cosmic DE (CachyOS/Arch)

[pixelydn]

Pre-Submission Checklist

• I have checked the existing/closed issues to ensure this issue has not already been reported.
• I am using the latest supported version of the browser.
• I have tried disabling all browser extensions/add-ons to rule out interference from these.
• I have tried clearing the browser cache and cookies.
• I have checked with a new profile and the issue still occurs.
• I have provided detailed step-by-step instructions on how to reproduce the issue.
• I have included relevant screenshots or console outputs.
• I have checked that this problem is not occurring in the latest version of Firefox.
• This issue is specific to this browser and does not occur in other browsers.

Bug Summary

The "Set as Default" button in Floorp settings corrupts ~/.config/mimeapps.list on Arch-Based ChachyOS with Cosmic DE. Instead of setting floorp.desktop, it incorrectly writes legacy userapp-Firefox-[random-ID].desktop entries into the [Default Applications] section, effectively reverting the default browser to a non-existent Firefox instance.

Reproduction Steps

1. On CachyOS with Cosmic DE, manually configure ~/.config/mimeapps.list to set floorp.desktop as the default for text/html, x-scheme-handler/http, and x-scheme-handler/https.
2. Verify via terminal that xdg-settings get default-web-browser returns floorp.desktop.
3. Open Floorp and navigate to Settings -> General.
4. Floorp reports it is "not the default browser". Click the "Make Default" button.
5. Open ~/.config/mimeapps.list and inspect the [Default Applications] section.

Expected Behavior

The mimeapps.list file should either remain unchanged since Floorp was already manually set as default or the keys will be updated to point to floorp.desktop.

Actual Behavior

The [Default Applications] section is immediately overwritten. The keys for text/html, http, and https are changed to point to userapp-Firefox-[random-ID].desktop. This causes the system default to break or revert to Firefox immediately.

Screenshots

Browser Version

Floorp version: 12.10.3 | Firefox version: 147.0.3

Operating System

CachyOS (Arch-Based) with COSMIC Desktop Enviroment

Additional Context

Thank you so much for helping.